US President Donald Trump increased pressure on Saudi Arabia on Wednesday to provide information on what happened to missing Saudi journalist Jamal Khashoggi and said he wanted to get the bottom of what he called "a very serious situation."

In a fresh clue, a pro-government Turkish daily on Wednesday published preliminary evidence from investigators it said identified a 15-member Saudi intelligence team involved in Khashoggi's unexplained disappearance on October 2.

Pressure appeared to be building worldwide on close US ally Saudi Arabia on the whereabouts of Khashoggi, a prominent critic of Saudi policies, who was last seen entering the Saudi consulate in Istanbul to get documents for his planned marriage.

Khashoggi's Turkish fiancée, Hatice Cengiz, who was waiting outside, said he never re-appeared. Turkish sources said they believe Khashoggi was killed inside the building and removed, allegations that Riyadh dismisses as baseless.

Asked in a telephone interview with Fox News Channel late on Wednesday whether Saudis were responsible for Khashoggi's disappearance or death, Trump said: "I guess you would have to say so far it's looking a little bit like that and we're going to have to see."

Earlier in the Oval Office, Trump told reporters he had raised Khashoggi's case with Saudi Arabia "at the highest level" and more than once in recent days.

"We're demanding everything," Trump said when asked if he was demanding information from the Saudis. "We want to see what's going on. It’s a very serious situation for us and for this White House ... We want to get to the bottom of it."

He said he and his wife, Melania, expect to invite Cengiz to the White House soon.

"People saw him go in and didn't see him come out. We're going to take a very serious look at it. It's a terrible thing," Trump said. "This is a bad situation. We cannot let this happen, to reporters, to anybody. We can't let this happen."

Increasing pressure on Trump to respond, a bipartisan group of US senators on Wednesday triggered a US investigation into Khashoggi's disappearance using a human rights law.

They asked Trump to determine whether "a foreign person is responsible for an extrajudicial killing, torture, or other gross violation of internationally recognised human rights" and report back in 120 days with a decision on the imposition of sanctions on whoever was responsible.

Asked about suggestions that Washington should consider blocking further arms sales to Saudi Arabia, Trump told Fox that would hurt US jobs.

"Frankly I think that would be a very very tough pill to swallow for our country. ... Before I discuss this I'd have to find out what happened," he said.

Former US Energy Secretary Ernest Moniz said he has suspended his role on the board of Saudi Arabia's planned mega city NEOM until more is known about Khashoggi.

After Trump spoke, the White House said national security adviser John Bolton and White House senior adviser Jared Kushner, Trump's son-in-law, had spoken to Saudi Crown Prince Mohammed bin Salman about the matter on Tuesday.

US Secretary of State Mike Pompeo followed up with his own call to the crown prince, who had forged close ties to the administration, especially Kushner.

"In both calls they asked for more details and for the Saudi government to be transparent in the investigation process. We will continue to monitor this situation and provide updates as available," White House spokeswoman Sarah Sanders said in a statement.

Khashoggi's fiancée Cengiz was surprised when an American friend told her of Trump's comment about inviting her to the White House, the friend told Reuters. "I think she's in shock, now even more," her friend said.

In an essay in the Washington Post on Tuesday, Cengiz implored the president and Melania Trump to "help shed light on Jamal's disappearance."

Trump made Saudi Arabia the first stop on his first foreign trip as president in May 2017, but in recent weeks has appeared to sour a bit on Riyadh, complaining directly to King Salman about the cost of American support for the Saudi military and for OPEC oil price increases.

SAUDI VISITORS

One of the 15 Saudis identified by name and photo in the report by Turkey's Sabah newspaper is a forensic expert who has worked at the Saudi Interior Ministry for 20 years, according to a LinkedIn profile and Saudi media, and serves on the board of the Saudi Society of Forensic Medicine.

Other names and photos of the 15, who Sabah said travelled on diplomatic passports, match officers in the Saudi Army and Air Force, as identified by previous Saudi media reports and in one case a Facebook profile.

The Saudi consulate referred Reuters to authorities in Riyadh who have not responded to questions about the 15 Saudis, who arrived in the city hours before Khashoggi disappeared on Oct. 2.

A Turkish security source had previously told Reuters that the 15 Saudis, including some officials, left Turkey later the same day.

The Washington Post reported on Tuesday that U.S. intelligence had intercepted the communications of Saudi officials discussing a plan to capture Khashoggi, citing an unidentified person familiar with the information.

On Wednesday, State Department spokesman Robert Palladino said the United States "had no advance knowledge" of such a plan.

The crown prince ordered an operation to lure Khashoggi back to Saudi Arabia from the United States and then detain him, the Washington Post reported on Wednesday, citing U.S. intelligence intercepts of Saudi officials discussing the plan.

The Post said it was not clear to officials with knowledge of the intelligence whether the Saudis discussed harming Khashoggi as part of the plan to detain him in Saudi Arabia.

Khashoggi left Saudi Arabia last year saying he feared retribution for his criticism of Riyadh over the Yemen war and its crackdown on dissent, and since then wrote columns for the Washington Post.

Britain's foreign minister called for urgent answers and the chairman of the U.S. Senate Foreign Relations Committee Bob Corker, who has seen classified intelligence on the case, said information pointed to Khashoggi being killed.

"To me ... it feels very much some nefarious activity has occurred by them. But I don't want to rush to judgment," Corker said on Tuesday.

US Vice President Mike Pence said on Wednesday the United States was ready to help in any way in the investigation.

Wednesday's Turkish media reports were the most detailed accounts yet of what they say is the Turkish government's investigation into Khashoggi's disappearance.

Sabah newspaper published the names and years of birth of the 15 Saudis it said arrived at Istanbul's Ataturk airport. Twelve of them arrived early on Tuesday, based on photos captured at passport control which it published. The 15 departed at four different times, Sabah reported.